WASHINGTON — Washington Navy Yard gunman Aaron Alexis left a note saying he was driven to kill by months of bombardment with extremely low-frequency radio waves, the FBI said Wednesday in a disclosure that explains the phrase he etched on his shotgun: “My ELF Weapon!”

Alexis did not target particular individuals during the Sept. 16 attack in which he killed 12 people, and there is no indication the shooting stemmed from any workplace dispute, said Valerie Parlave, assistant director in charge of the FBI’s Washington field office. Instead, authorities said, his behavior in the weeks before the shooting and evidence recovered from his hotel room, backpack and other belongings reveal a man increasingly in the throes of paranoia and delusions.

“Ultra-low frequency attack is what I’ve been subject to for the last 3 months, and to be perfectly honest that is what has driven me to this,” read an electronic document agents recovered after the shooting.

The attack came one month after Alexis had complained to police in Rhode Island that people were talking to him through the walls and ceilings of his hotel room and sending microwave vibrations into his body to deprive him of sleep.

On his shotgun, he had scrawled “My ELF Weapon!” – an apparent reference to extremely low-frequency waves – along with “End to the Torment!” “Not what yall say” and “Better off this way,” the FBI said.

Alexis, a 34-year-old former Navy reservist and computer technician for a government contractor, used a valid badge to get into the Navy Yard and opened fire inside a building with the Remington shotgun, which he had legally purchased in Virginia two days earlier. He also used a 9 mm handgun that he took from a security guard, a weapon found near Alexis’ body. He was killed in the building by a U.S. Park Police officer following a rampage and shootout with police that the FBI said lasted more than an hour.

“There are indicators that Alexis was prepared to die during the attack and that he accepted death as the inevitable consequence of his actions,” Parlave said.

Surveillance video released by the FBI on Wednesday shows Alexis pulling his rental car into a garage, walking into the building with a bag and then skulking down a corridor with a shotgun, ducking and crouching around a corner and walking briskly down a flight of stairs. The video does not show him actually shooting anyone.

A timeline issued by the FBI shows Alexis started the rampage on the building’s fourth floor and then moved down to the third and first floors. He ultimately returned to the third floor, where he was killed around 9:25 a.m. FBI Director James Comey has said there’s no evidence that Alexis shot down into the atrium despite earlier accounts from witnesses at the scene.

Alexis had started a job as a contractor in the building just a week before.

Although there was a “routine performance-related issue addressed to him” on the Friday before the Monday morning shooting, “there is no indication that this caused any sort of reaction from him,” Parlave said.

“We have not determined there to be any previous relationship between Alexis and any of the victims,” she said. “There is no evidence or information at this point that indicates he targeted anyone he worked for or worked with. We do not see any one event as triggering this attack.”

Defense officials have acknowledged that a lot of red flags were missed in Alexis’ background, allowing him to maintain a secret-level security clearance and access to a Navy installation despite a string of behavioral problems and brushes with the law.

He worked for The Experts, a Florida-based computer firm that was a Hewlett-Packard subcontractor. Hewlett-Packard said Wednesday that it was severing ties with The Experts, accusing the company of failing to respond adequately to Alexis’ mental problems. A spokesman for The Experts didn’t immediately return a call seeking comment.

At the Pentagon, Deputy Defense Secretary Ash Carter said the department will review base safety procedures and the security clearance process.

“Bottom line is, we need to know how an employee was able to bring a weapon and ammunition onto a DoD installation, and how warning flags were either missed, ignored or not addressed in a timely manner,” Carter said.

Navy Secretary Ray Mabus has recommended that the department require that all police reports – not just arrests or convictions – be included in background checks.